Jealousy is (justly) perceived as a form of transformed aggression. To direct it at the narcissist's female partner (who stands in for the primary object, his Mother) is to direct it at a forbidden object. It triggers a strong feeling of imminent punishment - a likely abandonment (physical or emotional).
But this is merely the "surface" conflict. There is yet another layer, much harder to reach and to decipher.
To feed his envy, the narcissist exercises his imagination. He imagines situations, which justify his negative emotions. If his mate is sexually promiscuous this justifies romantic jealousy â he unconsciously "thinks".
The narcissist is a con artist. He easily substitutes fiction for truth. What commences as an elaborate daydream ends up in the narcissist's mind as a plausible scenario. But, then, if his suspicions are true (they are bound to be - otherwise, why is he jealous?), there is no way he can accept his partner back, says the narcissist to himself. If she is unfaithful - how could the relationship continue?
Infidelity and lack of exclusivity violate the first and last commandment of narcissism: uniqueness.
(From the book "Malignant Self-love: Narcissism Revisited" by Sam Vaknin - Click on this link to purchase the print book, or 16 e-books, or 2 DVDs with 12 hours of video lectures on narcissists, psychopaths, and abuse in relationships: http://www.narcissistic-abuse.com/thebook.html)